A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a pattern provided by a patterning device (e.g., a reticle pattern), may be used to generate a circuit pattern to be formed on an individual layer of the IC. The circuit pattern can be transferred onto a target portion (e.g. comprising part of a die, one die or several dies) on a substrate (e.g. a silicon wafer). Transfer of the circuit pattern is typically via imaging of the pattern onto a layer of radiation-sensitive material (e.g., photo-activated resist or photoresist) provided on the substrate, using a projection system. A beam of radiation is patterned by having that beam traverse the patterning device, and is projected by the projection system onto the target portion on the substrate, such as to image the desired pattern in the resist. A lithographic printing process further includes a development of the resist layer after exposure such as to generate printed features, which may be features of resist material, or spaces in resist material. The resist material may serve as an etch mask for an underlying layer to be patterned by etching.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
There is a continuing desire to be able to generate patterns with finer resolution. In general, shorter wavelength radiation may be used in order to achieve a finer resolution pattern. Step and scan systems are becoming resolution limited, particularly using radiation having 193 nm wavelength. Resolution has been extended using immersion lithography which allows a numerical aperture increase to approximately 1.56 NA. This will support 32 nm (half pitch) resolution. To go to higher resolution, particularly using 193 nm wavelength illumination, will require the development of new patterning techniques.
A patterning technique that has been developed to try to afford etching of smaller features employs the use of a hard mask provided in between a substrate layer that is to be patterned and a layer of resist. The hard mask is more resistant than the resist material to the etching conditions required to transfer the intended pattern into the underlying substrate, which therefore avoids problems associated with the resist being etched at a faster rate than the substrate during pattern transfer. In spite of the development of such methods employing hard masks, limitations on the minimum size of features that can be etched into a substrate are imposed by, for example, the wavelength of radiation used for the pattern imaging. Based on lithography tool parameters such as numerical aperture, a minimum pitch at which adjacent features can be printed on a substrate (“minimum feature pitch”) is limited for a given wavelength of radiation used. The minimum pitch used in lithography reticles (taking into account the demagnification from the reticle to the substrate) is generally selected according to the minimum feature pitch printable using the lithography tool employing the reticle, since any reticle features arranged at corresponding smaller pitches would not print out on the substrate. Thus, for a given lithography apparatus, a reticle or set of reticles used to produce a given patterned beam of radiation, is typically configured with reticle features that are designed to produce structures in a substrate that have a feature pitch equal to or larger than the minimum feature pitch printable by the lithography apparatus.
In view of the above limitations, as the required separation (minimum feature pitch required) between features shrinks in advanced electronic technologies, it becomes increasingly difficult to fabricate a device using conventional single exposure processes, in which a substrate is exposed to a patterned beam of radiation in a single exposure. For example, for sub-90 nm device size, it is increasingly difficult to pattern features using 248 or 193 nm radiation. In order to increase the amount of structures on a substrate, so that structures can be spaced at a distance below the minimum feature pitch associated with a lithography tool used to pattern the substrate, double exposure techniques have been developed.
In one example of a current double exposure dual trench printing process, a layer of resist overlaying a layer of hard mask material deposited on a substrate is exposed to a first dose of patterned radiation constituting an image of a reticle pattern. Exposed regions of the resist are then removed, followed by a hard mask etch step, to form a first series of spaces in the hard mask. A further layer of resist is then provided to coat the layer of hard mask material, followed by displacement of the patterning reticle by a predetermined distance. Subsequent patterning and etching of respectively the resist layer and the hard mask layer to provide a second series of spaces results in new spaces disposed in interleaved positions between the first series of spaces. Residual resist is then selectively removed leaving the patterned hard mask with both the first and second series of spaces formed therein. The pattern defined by the hard mask can then be etched into the underlying substrate. In this way, a reticle configured to produce a feature pitch D in a single exposure, can be used to pattern and form a hard mask and thereby a substrate, with a feature pitch less than D, for example D/2.